Magnetic rubber molded articles made by crosslinking a magnetic rubber composition containing a rubber and a magnetic powder have been used for various applications. One suitable application of a magnetic rubber molded article is a magnetic encoder, which is produced by magnetizing the magnetic rubber molded article. Here, a suitable rubber is an oil resistant rubber such as a nitrile rubber and an acrylic rubber. As a magnetic powder, a ferrite magnetic powder, a rare-earth magnetic powder and so on are properly used, depending on required performance. A magnetic encoder produced using such a magnetic rubber molded article is extensible even when it contains a large amount of a magnetic powder, and have been, therefore, widely used in applications in which flexibility is required. However, a magnetic encoder using such a magnetic rubber molded article is sometimes insufficiently friction-resistant and has been thus used for limited applications.
Patent Reference No. 1 has described a magnetic encoder made by forming an annular product from a magnetic material containing a magnetic powder and a binder, which is then circumferentially multipole-magnetized, wherein the binder is a thermosetting resin and the binder accounts for 10 to 70% by volume of the magnetic material. The reference has described that the magnetic encoder has a large magnetic flux density, is resistant to breakage and detachment from a mounting plate of a magnet unit, exhibits high productivity, is highly reliable and is inexpensive. This magnetic encoder is, however, substantially inextensible and could not been, therefore, used for applications requiring flexibility.
Patent Reference No. 2 has described a bond magnet in which a rare-earth magnetic powder is dispersed in a rubber binder, wherein the rare-earth magnetic powder is coated by an antirust agent and an epoxy resin. For producing the bond magnet, a rare-earth magnetic powder is pre-coated by a film made of an antirust agent and an epoxy resin, and then mixed with a rubber binder and a curing agent. A bond magnet described in an example therein contains a 0.12-fold by weight of an epoxy resin based on a rubber binder. Furthermore, there is described that in the bond magnet, oxidation of a rare-earth magnetic powder can be inhibited. This magnetic encoder is, however, insufficiently abrasion-resistant.